Microlithography (also called photolithography or simply lithography) is a technology for the fabrication of integrated circuits, liquid crystal displays and other microstructured devices. The process of microlithography, in conjunction with the process of etching, is used to pattern features in thin film stacks that have been formed on a substrate, for example a silicon wafer. In general, at each layer of the fabrication, the wafer is first coated with a photoresist which is a material that is sensitive to radiation, such as deep ultraviolet (DUV) light. Next, the wafer with the photoresist on top is exposed to projection light through a mask in a projection exposure apparatus. The mask contains a circuit pattern to be projected onto the photoresist. After exposure the photoresist is developed to produce an image corresponding to the circuit pattern contained in the mask. Then an etch process transfers the circuit pattern into the thin film stacks on the wafer. Finally, the photoresist is removed. Repetition of this process with different masks results in a multi-layered microstructured component.
A projection exposure apparatus typically includes an illumination system, a mask alignment stage for aligning the mask, a projection lens and a wafer alignment stage for aligning the wafer coated with the photoresist. The illumination system illuminates a field on the mask that may have the shape of a rectangular slit or a narrow ring segment, for example.
In current projection exposure apparatus a distinction can be made between two different types of apparatus. In one type each target portion on the wafer is irradiated by exposing the entire mask pattern onto the target portion in one go; such an apparatus is commonly referred to as a wafer stepper. In the other type of apparatus, which is commonly referred to as a step-and-scan apparatus or simply scanner, each target portion is irradiated by progressively scanning the mask pattern under the projection light beam in a given reference direction while synchronously scanning the substrate parallel or anti-parallel to this direction. The ratio of the velocity of the wafer and the velocity of the mask is equal to the magnification β of the projection lens, which is usually smaller than 1, for example β=¼ or β= 1/100.
It is to be understood that the term “mask” (or reticle) is to be interpreted broadly as a patterning device. Commonly used masks contain transmissive or reflective patterns and may be of the binary, alternating phase-shift, attenuated phase-shift or various hybrid mask type, for example. However, there are also active masks, e.g. masks realized as a programmable mirror array. An example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. More information on such mirror arrays can be gleaned, for example, from U.S. Pat. No. 5,296,891 and U.S. Pat. No. 5,523,193. Also programmable LCD arrays may be used as active masks, as is described in U.S. Pat. No. 5,229,872. For the sake of simplicity, the rest of this text may specifically relate to apparatus including a mask and a mask stage; however, the general principles discussed in such apparatus should be seen in the broader context of the patterning device as hereabove set forth.
One of the aims in the development of projection exposure apparatus is to be able to lithographically generate structures with smaller and smaller dimensions on the wafer. Small structures lead to high integration densities, which generally has a favorable effect on the performance of the microstructured components produced with the aid of such apparatus.
The size of the structures which can be generated often depends primarily on the resolution of the projection objective being used. Since the resolution of projection objectives is inversely proportional to the wavelength of the projection light, one way of increasing the resolution is to use projection light with shorter and shorter wavelengths. The shortest wavelengths currently used are 248 nm, 193 nm or 157 nm and thus lie in the (deep) ultraviolet spectral range.
Another way of increasing the resolution is based on the idea of introducing an immersion liquid with a high refractive index into an immersion interspace, which remains between a last lens on the image side of the projection objective and the photoresist or another photosensitive layer to be exposed. Projection objectives which are designed for immersed operation, and which are therefore also referred to as immersion objectives, can achieve numerical apertures of more than 1, for example 1.4 or even higher.
The correction of image errors (i.e. aberrations) is becoming increasingly important for projection objectives with very high resolution. Different types of image errors usually involve different correction measures.
The correction of rotationally symmetric image errors is comparatively straightforward. An image error is referred to as being rotationally symmetric if the wavefront deformation in the exit pupil is rotationally symmetric. The term wavefront deformation refers to the deviation of a wave from the ideal aberration-free wave. Rotationally symmetric image errors can often be corrected, for example, at least partially by moving individual optical elements along the optical axis.
Correction of those image errors which are not rotationally symmetric can be more difficult. Such image errors occur, for example, because lenses and other optical elements heat up rotationally asymmetrically. One image error of this type is astigmatism, which may also be encountered for the field point lying on the optical axis.
A major cause for rotationally asymmetric image errors is a rotationally asymmetric, in particular slit-shaped and/or off-axis, illumination of the mask, as is typically encountered in projection exposure apparatus of the scanner type. The slit-shaped illuminated field causes a non-uniform heating of those optical elements that are arranged in the vicinity of field planes. This heating results in deformations of the optical elements and, in the case of lenses and other elements of the refractive type, in changes of their index of refraction. If the materials of refractive optical elements are repeatedly exposed to the high energetic projection light, also permanent material changes are observed. For example, a compaction of the materials exposed to the projection light sometimes occurs, and this compaction results in local and permanent changes of the index of refraction.
The heat induced deformations and index changes alter the optical properties of the optical elements and thus cause image errors. Heat induced image errors often have a twofold symmetry. However, image errors with other symmetries, for example threefold or fivefold, or image errors characterized by completely asymmetric wavefront deformations are also frequently observed in projection objectives. Completely asymmetric image errors are often caused by material defects which are statistically distributed over the optical elements contained in the projection objective.
Another major cause for rotationally asymmetric image errors are certain asymmetric illumination settings in which the pupil plane of the illumination system is illuminated in a rotationally asymmetric manner. Important examples for such settings are dipole settings in which only two poles are illuminated in the pupil plane. With such a dipole setting, also the pupil planes in the projection objective contain only two illuminated regions. Consequently, the lenses arranged in or in the vicinity of such an objective pupil plane are exposed to a rotationally asymmetric intensity distribution which gives rise to rotationally asymmetric image errors. Also quadrupol settings often produce rotationally asymmetric image errors, although to a lesser extent than dipole settings.